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A lot of runners think they’re “lean enough” when they actually aren’t because they assume their diet is “good enough” when it’s actually not.

Everyone knows it’s important to be lean if you want to swim, bike and run fast. But how important really is it?

Written by: Matt Fitzgerald

Swiss exercise scientists recently focused their attention on 42 recreational female runners who participated in a half marathon. They quizzed the runners on their training habits and also took various anthropometric measurements and then attempted to correlate this data with their race finish times.

The researchers found that body fat percentage was among the best predictors of race finish times—an even better predictor than training volume. This finding isn’t too surprising. We all know that being lean is critical to running performance. We also know that fitness is critical to running performance, and as fitness goes up, body fat percentage tends to come down. Among recreational runners, there tend to be large differences in leanness, and it’s only to be expected that the leanest recreational runners will perform best in races.

Among elite-level runners it’s a different story. All elite runners are very lean, and the small differences in body fat percentages have little correlation with differences in performance. Within the special population of elite runners, it’s small differences in VO2max, maximum speed, and running economy that determine who wins and who loses.

Except that nothing I said in the previous paragraph is true. Believe it or not, differences in body fat percentage predict races times as well in elite runners as they do among recreational female runners. This was shown in a 2009 study involving 24 elite runners in Ethiopia. Skinfold measurements were used to estimate body fat percentage in 12 male and 12 female athletes. These estimates were then compared to the runners’ individual race performances. The researchers found an 80 percent correlation between skinfold measurements and race times in the men and a 78 percent correspondence in the women. All of these runners were very lean and very light, but the leanest among them were the fastest.

At every level of the sport, leanness is as important as aerobic capacity, speed, and running economy. And even at the elite level, it seems, some runners could get faster by getting leaner. A good case in point is Chris Solinsky, who made a quantum leap in performance last year when he broke the American record for 10,000m (26:59.60) and lowered his 5000m PR from 13:18.41 to 12:55.53. That leap coincided with a visible leaning out that was widely commented on at the time. While it’s impossible to separate the direct effect of Solinsky’s fat loss from those of the training that contributed to the fat loss on his performance, I think it’s reasonable to speculate that the fat loss did have a direct effect.

Many high-level runners who look very lean assume they are “lean enough”, but there is cause to believe that some of these runners could perform better by getting even leaner. What is certain is that leanness is critical enough to performance that every serious runner should monitor his body fat percentage as closely as he monitors his training.

Naturally, there are right and wrong ways to get leaner. Eating too little is definitely the wrong way. Not only will it fail to make you leaner by causing you to lose muscle along with (or even to some degree instead of) fat, but it will also sabotage your training by leaving your muscles under-fueled for maximum performance. The right ways to get leaner are to sensibly increase training volume, add more high-intensity running to your training, lift weights, and clean up your diet.

The last of these measures probably has the greatest potential to yield results in most cases. A lot of runners think they’re “lean enough” when they actually aren’t because they assume their diet is “good enough” when it’s actually not. If you look closely at your diet, you will probably find some flab that is very likely keeping a little extra flab on your body. Even small improvements could yield a small reduction in your measured body fat percentage, which may in turn result in your own Solinsky-style breakthrough.